

Ml AM IK III 

 DEVELOPM1 



I. I'.MI'.KVOLOGY 



Ovum. The ovum of an insect, as of any other animal, i^ a -ingle 

 i go), with a large nucleus (germinal vesicle), a Large fffttfco/ftt, 

 nutritive matter, or yolk (dculophism), contained in cytoplasm, and a 

 cell wall i;//c///;/r membrane) secreted by the ovum. 

 The eggshell, or chorion , is secreted around the 

 ovum by surrounding ovarian cells. 



Maturation. As a preparation for fertilization 

 the germinal vesicle divides twice, forming two 

 polar bodies, and as the first of these bodies may 

 itself divide, there result four cells; three of these, 

 however the polar bodies are minute and rudi- 

 mentary. 



The>e phenomena of ovogenesis are paralleled 



in the development of the spermatozoa, or sperma- 



: for the primary spermutocyte gives rise 



- t'otnlary spermatocytes, and these to four 



ach of which forms a spermatozoon. 

 By means of this maturation process the number 

 iiromosoincs in the egg- nucleus is reduced to 

 half the number normal for somatic cells (body 

 <listinguished from germ cells). A simi- 

 lar reduction occurs also during the develop- 

 ment of the spermatozoon, and when sperm-nucleus 

 and cgg-n itclcus unite, the resulting nucleus con- 

 the normal number of chromosomes. The 

 meaning of these reduction phenomena highly 

 important, from the standpoint of heredity is a 

 much debated subject. 



Fertilization. As the eggs pass through the 

 hey are capable of being fertilized by 



previously stored in the seminal receptacle. Through 

 micro p\le of the chorion one or more spermatozoa enter and a 



129 



P r 



FIG. 190. ^Sagittal 

 section of egg of fly, 

 Musca, in process of fer- 

 tilization, c, chorion; d, 

 dorsal; m. micropyle, 

 with gelatinous exuda- 

 tion; p, male and female 

 pronuclei, before union; 

 pb, polar bodies; pr, 

 peripheral protoplasm; 

 v, ventral; vt, vitelline 

 membrane; y, yolk. 

 After H E N K i N G and 

 BLOCHMANN. 



